Most manufacturers of gas turbine engines are evaluating advanced multi-walled, thin-walled superalloy gas turbine airfoils (i.e. turbine blade or vane) which include intricate air cooling channels to improve efficiency of airfoil internal cooling to permit greater engine thrust and provide satisfactory airfoil service life. U.S. Pat. Nos. 5,295,530 and 5,545,003 describe advanced multi-walled, thin-walled turbine blade or vane designs which include intricate air cooling channels to this end.
In casting hollow gas turbine engine blades and vanes (airfoils) having internal cooling passageways, a fired ceramic core is positioned in a ceramic investment shell mold to form internal cooling passageways in the cast airfoil. The fired ceramic core used in investment casting of hollow airfoils typically has an airfoil-shaped region with a thin cross-section leading edge region and trailing edge region. Between the leading and trailing edge regions, the core may include elongated and other shaped openings so as to form multiple internal walls, pedestals, turbulators, ribs and similar features separating and/or residing in cooling passageways in the cast airfoil.
The ceramic core typically is formed to desired core configuration by injection molding, transfer molding or pouring of an appropriate fluid ceramic core material that includes one or more ceramic powders, a binder, and optional additives into a suitably shaped core molding die. After the green molded core is removed from the die, it is subjected to firing at elevated (superambient) temperature in one or more steps to remove the fugitive binder and sinter and strengthen the core for use in casting metallic material, such as a nickel or cobalt base superalloy typically used to cast single crystal gas turbine engine blades and vanes (airfoils).
The fired ceramic core then is used in manufacture of the shell mold by the well known lost wax process wherein the ceramic core is placed in a pattern molding die and a fugitive pattern is formed about the core by injecting under pressure pattern material, such as wax, thermoplastic and the like, into the die in the space between the core the inner die walls. The pattern typically has an airfoil-shaped region with a thin cross-section trailing edge region corresponding in location to trailing edge features of the core.
The fugitive pattern with the ceramic core therein is subjected to repeated steps to build up the shell mold thereon. For example, the pattern/core assembly is repeatedly dipped in ceramic slurry, drained of excess slurry, stuccoed with coarse ceramic stucco or sand, and then air dried to build up multiple ceramic layers that form the shell mold on the assembly. The resulting invested pattern/core assembly then is subjected to a pattern removal operation, such as steam autoclaving, to selectively remove the fugitive pattern, leaving the shell mold with the ceramic core located therein. The shell mold then is fired at elevated temperature to develop adequate shell mold strength for metal casting.
Molten metallic material, such as a nickel or cobalt base superalloy, is cast into a preheated shell mold and solidified to produce an equiaxed grain, columnar grain or single crystal airfoil. The resulting cast airfoil includes the ceramic core therein so as to form internal cooling passageways upon removal of the core. The core can be removed by leaching or other conventional techniques, leaving a hollow cast metallic airfoil.